As the sizes and spaces of features in a semiconductor structure decrease, it becomes harder to form the densely arranged features. One of the reasons is the resolution limitation of lithography processes. To solve this problem, multiple patterning technologies, such as double patterning technology, are proposed. An example is pitch splitting. The densely arranged features are divided into two interleaving sets, and the two sets of the features are formed using two separate masks. More specifically, one half of the features are formed using one of the masks. The other half of the features then are formed using the other one of the masks in a manner that each of these features is located between two previously formed features. As such, the densely arranged features can be formed by less densely arranged patterns on the masks, wherein the pitch of which is twice of the pitch of the features to be formed.